This invention relates to a paper container for containing liquid such as milk and juice and sold at retail. More particularly, the invention relates to a paper container for liquid which is formed by folding a thick paper coated with polyethylene, aluminum and the like along several vertical and horizontal folding line grooves provided on the paper.
Conventional containers of this type are usually formed by providing folding line grooves on a bare thick paper and thereafter applying polyethylene, aluminum and the like on the paper in several layers and bonding thereof.
When such thick paper is folded along the folding line grooves to build up a container, splits and pin holes are likely to occur in the area near the folding line grooves.
Regarding the structure of folding lines at the area near the instersection of the vertical and horizontal folding line grooves, several proposals have been made. According to such proposed structure of folding lines, the folding lines have interrupted portions and, at the interrupted portions, the folding lines are replaced with two side fold lines running in the main direction. Due to the foregoing, the thick paper is more sharply folded at the interrupted portions of the folding lines and the expansion of the thick paper becomes small at the folded portion, and therefore the tensile stress occurred to the ourter side of the thick paper is eased.
When a container is built up by folding a composite thick paper along several folding line grooves formed thereon, the following tendency is likely to occur. That is, when the paper is folded by 180.degree. along one of the folding line grooves, the inner side of the center line of the thickness of the thick paper is contracted, while the outer side is expanded. Because of the foregoing, a compression pressure is applied to the inner side and a tensile stress is applied to the outer side.
Although there is almost no problem on the inner portion which receives the compression pressure, the outer portion is tended to have splits and pin holes at the layer portion due to the tensile stress. This is especially true when an aluminum layer is formed. Although such splits and pin holes are not produced by only one trial of 180.degree. folding, when the folded portion of the thick paper is further folded by 180.degree. in the direction perpendicular thereto or when a portion where two sheets of thick paper overlapped each other is folded twice in the manner as just mentioned, the tensile stress applied to the layer portion situated at the outer side becomes larger and splits and pin holes are likely to take place.
In the development view of a container shown in FIG. 11, the intersecting portion between the folding line grooves 2 and 8 are the intersecting portion between the folding line grooves 3 and 8 are formed of two sheets of thick paper overlapped each other, and these overlapped thick paper portions are once folded by 180.degree. along the horiziontal folding line groove 8 and thereafter folded again by 180.degree. in the perpendicular direction to build up a container. FIG. 12 shows the configuration of this portion, and FIGS. 13 and 14 are sectional views thereof.
In FIG. 13, the paper layers 105-103-110 hold at the inner sides thereof four paper layers 106, 107, 108 and 109, and are folded by 180.degree.. Accordingly, the maximum tensile stress is produced at the outer side of the paper layer 3. Further, since this portion is already folded by 180.degree. as denoted by 109, 100 in FIG. 12, a secondary tensile stress is produced.
From the foregoing, the widths of folding line grooves on the paper layers 105, 103, 110 of FIG. 10 are desirably five times of the thickness of a sheet of paper or single paper layer.
In order to keep the width of the folding line grooves narrow, the thicknesses of the papers 105 through 110 should be formed thin.
As described in the foregoing, if the widths of the folding line grooves are formed wide or otherwise if the thicknesses of the paper layers around the folding line grooves are formed thin according to the configuration of the folding paper layers, i.e., the number of layers at the portion where the papers are folded and to the folding angles thereof, the tensile stress produced at the outer side of the paper layers can be eased.